Use BranchInst::getSuccessor().

This is more resilient to changes in LLVM.

Author: vext01

llvm/lib/YkIR/YkIRWriter.cpp

@@ -459,16 +459,17 @@ class YkIRWriter {
       //
       // Note that in LLVM IR, the operands are ordered (despite the order they
       // appear in the language reference): cond, if-false, if-true. We
-      // re-order those during lowering to avoid confusion.
+      // use `getSuccessor()`, so as to re-order those during lowering to avoid
+      // confusion.
       //
       // num_operands:
       OutStreamer.emitInt32(3);
       // OPERAND 0: condition.
       serialiseOperand(I, VLMap, I->getOperand(0));
       // OPERAND 1: block to go to if true.
-      serialiseOperand(I, VLMap, I->getOperand(2));
+      serialiseOperand(I, VLMap, I->getSuccessor(0));
       // OPERAND 2: block to go to if false.
-      serialiseOperand(I, VLMap, I->getOperand(1));
+      serialiseOperand(I, VLMap, I->getSuccessor(1));
     }
     InstIdx++;
   }